Process printing developed as a solution for printing many copies of an item, including glossy magazines, posters and so forth, in a single process on substantially hard and flat surfaces. During process printing, a full or multicolor original is reproduced through the use of several (usually between two and four) halftone plates. The colors typically used are cyan, magenta, yellow, and black, which are known as CMYK process colors. Process printing has allowed for a variety of vibrant and vivid colors to be used in the print and publication industries. Unfortunately, process printing cannot be readily applied to print on fabrics because of the technical challenges and commercial difficulty that arise in precisely positioning process color dots on a moving piece of stretching cloth. Previous attempts at process printing on fabrics typically have required too much time and effort to achieve a satisfactory product.
U.S. Patent Appl. No. 2005/0025520 to Murakami discusses an image forming apparatus that can produce spherically-shaped toner such that any toner that is not transferred during the printing process can be more easily removed from the latent image bearing member. U.S. Patent Appl. No. 2002/0083855 to Samworth discusses methods of creating printing plates with ink cells having both solid and halftone areas. However, both Murakami and Samworth fail to contemplate solutions to the problems associated with process printing.
One known solution for fabric printing is to use conventional sublimation technology, in which all of the CMYK process colors are printed on a donor paper in a single pass. The image can then be transferred from the donor paper to a target fabric using heat and pressure, i.e. sublimation. Although sublimation printing works reasonably well for certain images and fabrics, the sublimation process can be difficult to employ because different types of fabrics, and even different pieces or constructions of the same fabric, can have different reactions to the various dyes and inks. In addition, sublimation printing on fabrics generally is limited to relatively low resolutions (e.g., less than 400 dpi) because the colored dots tend to expand into one another resulting in dot gain. Dot gain can occur at each place where ink is placed on the paper, and results in a dot size that is larger than the specified dot size. For example, as a paper absorbs fountain pen ink, the ink spreads from whatever lines are drawn. Depending on the absorbency of the paper, the distance that the ink could spread can vary. Thus, while a printer might specify a specific dot size, the dot as printed can sometimes be 15% to 20% larger.
While dot gain might itself be insignificant, when four layers of a color separation are combined into one print, the dot gain can increase the measured tint value during prepress, plate making, printing and transfer, substantially change the final color(s) of the image, and usually degrade the image quality. Dot gain is especially problematic for sublimation printing, in which the massive dot gain that occurs during the gas transfer of dye from the donor paper to the receiving fabric can cause the dots to overlap at about 50% saturation, and therefore negate the available colors produced by process color printing. Dot gain makes images look darker than they should, and when printing in process color, can cause unwanted color shifts and loss of subtlety in photographic prints. Furthermore, dot gain in standard four color process sublimation reduces the colors available to less than a commercially viable palette.
Although simple to understand, dot gain is an extremely difficult problem to address. Among other things, the amount of dot gain during sublimation printing can vary in different stages and for a variety of reasons, including differences in donor papers, inks and final substrates. Although standard printing technology has developed compensation curves and techniques for dealing with relatively small dot gain, such as less than 20%, even small dot gain can be too much for conventional technology to deal with.
One solution to compensate for dot gain is to print using smaller dots on any given fabric as shown in U.S. Pat. No. 7,073,902 to Codos et al. The Codos solution is problematic because the dot gain percentage increases with the number of dots printed, which can result in undesirable dot gain especially for high resolution printing.
Thus, there is still a need for apparatus and methods that reduce dot gain, and thereby allow for, among other things, high resolution sublimation printing on a variety of fabrics.